Strained metal oxide semiconductor (SMOS) processes are utilized to increase transistor (MOSFET) performance by increasing the carrier mobility of silicon, thereby reducing resistance and power consumption and increasing drive current, frequency response and operating speed. Strained silicon is typically formed by growing a layer of silicon on a silicon germanium substrate or layer.
The silicon germanium lattice associated with the silicon germanium substrate is generally more widely spaced than a pure silicon lattice, with spacing becoming wider with a higher percentage of germanium. Because the silicon lattice aligns with the larger silicon germanium lattice, a tensile strain is created in the silicon layer. The silicon atoms are essentially pulled apart from one another.
Relaxed silicon has a conductive band that contains six equal valence bands. The application of tensile strain to the silicon causes four of the valence bands to increase in energy and two of the valence bands to decrease in energy. As a result of quantum effects, electrons effectively weigh 30 percent less when passing through the lower energy bands. Thus, the lower energy bands offer less resistance to electron flow. In addition, electrons meet with less vibrational energy from the nucleus of the silicon atom, which causes them to scatter at a rate of 500 to 1000 times less than in relaxed silicon. As a result, carrier mobility is dramatically increased in strained silicon compared to relaxed silicon, providing an increase in mobility of 80% or more for electrons and 20% or more for holes. The increase in mobility has been found to persist for current fields up to 1.5 megavolts/centimeter. These factors are believed to enable a device speed increase of 35% without further reduction of device size, or a 25% reduction in power consumption without a reduction in performance.
Conventional SOI substrates have included a strained silicon layer above a buried oxide layer above a base layer. The buried oxide layer can be formed in a variety of processes including deposition above the base layer or by doping the base layer with oxygen. The strained semiconductor layer can be formed by providing a silicon germanium layer having a composition of Si(1-x)Gex, where x is approximately 0.2, and is more generally in the range of 0.1–0.3. The silicon germanium layer can be deposited by chemical vapor deposition using silane and germane. The concentration of germane can be decreased as deposition occurs so that the upper most part of the silicon germanium layer is mostly or all silicon.
The use of germanium in SMOS processes can cause germanium contamination problems for IC structures, layers, and equipment. In particular, germanium outgassing or outdiffusion can contaminate various components associated with the fabrication equipment and integrated circuit structures associating with the processed wafer. Further, germanium outgassing can negatively impact the formation of thin films. In addition, germanium outdiffusion can cause germanium accumulation or “pile-up” at the interface of the liner, thereby causing reliability issues for the STI structure.
Germanium outgassing can be particularly problematic at the very high temperatures and HCI ambient environments associated with the liner of a shallow trench isolation (STI) structure. For example, conventional STI liner oxide processes can utilize temperatures of approximately 1000° C. which enhance germanium outgassing.
Thus, there is a need for a strained semiconductor substrate which can be formed without using germanium. Further still, there is a need for a process of forming high quality SMOS substrates. Further still, there is a need for an SMOS wafer formation process that does not require strained layer deposition. Yet further, there is a need for a substrate that is not as susceptible to germanium outgassing. Further still, there is a need for new process of forming strained semiconductor layers. Even further, there is a need for a wafer process that enhances and/or increases the longevity of the strained characteristic of a layer.